Amid rising global energy demands, interest is growing in the production of fuels and chemicals from renewable resources. Petroleum consumption reached 37.1 quadrillion BTU in the United States in 2008, of which a large majority (71%) was liquid fuel in the transportation sector. Petroleum and natural gas account for 99% of the feedstocks for chemicals such as plastics, fertilizers and pharmaceuticals in chemical industry (McFarlane et al.,“Survey of Alternative Feedstocks for Commodity Chemical Manufacturing.” Oak Ridge National Laboratory, 2007). Considering rapidly increasing world population and exhaustion of fossil fuels, the development of sustainable processes for energy and carbon capture (ECC) to produce fuels and chemicals is crucial for the human society.
In addition to increasing energy demands, renewable energy resources are of interest to address growing environmental issues. According to the United States Energy Information Administration (Serferlein, “Annual Energy Review, USEIA 2008), world energy-related CO2 emissions in 2006 were 29 billion metric tons, which is an increase of 35% from 1990. Accelerating accumulation of atmospheric CO2 is not only due to increased emissions from world growth and intensifying carbon use, but also from a possible attenuation in the efficiency of the world's natural carbon sinks (Raupach et al., Proc Natl Acad Sci USA, 104:10288-10293, 2007). As a result, atmospheric levels of CO2 have increased by ˜25% over the past 150 years. Thus, it has become increasingly important to develop new technologies to reduce CO2 emissions.
Previous methods of producing renewable energies have involved converting terrestrial plant biomass into biochemicals. However, these methods present undesirable complications, such as harsh chemical pretreatments of the biomass resulting in toxic byproducts and large land-use requirements to grow the plants. Photosynthetic microorganisms possess many advantages over traditional terrestrial plants with regard to biochemical production. For example, the photosynthetic efficiency of photosynthetic microorganisms is higher than plants, and photosynthetic microorganisms can be cultivated in locations that do not compete with traditional agricultural crops (Scharlemann et al., Science, 281:237-240, 2008).
An example of a photosynthetic microorganism with potential for biochemical production is cyanobacteria. Cyanobacteria are collectively responsible for almost 50% of global photosynthesis and are found in a wide range of environments (Field et al., Science, 281:237-240, 1998). While cyanobacteria have many similar features with algae in this context, many cyanobacterial species feature simpler genetic structures and faster growth rates (Ruffing, Bioeng Bugs, 2:136-149, 2011). As a result, genetic engineering methods for cyanobacteria are also more advanced in terms of genetic manipulation efforts than those for algae (Golden et al., Methods Enzymol, 153:215-231, 1987; Huang et al., Nucleic Acids Res, 38:2577-2593, 2010; and Heidorn et al., Methods Enzymol , 497:539-579, 2011).
Cyanobacteria have the biochemical machinery required to fix CO2, but lack the critical components to generate fuels and chemicals efficiently. Thus to produce valuable chemicals, cyanobacteria host strains must be equipped with new biosynthetic pathways (Keasling, ACS Chem Biol, 3:64-76, 2008; Ducat et al., Trends Biotechnol, 29:95-103, 2011; and Machado and Atsumi, J Biotechnol, 2012). Unfortunately, this approach in cyanobacteria is significantly less developed compared to a model organism such as Escherichia coli. Further, results in E. coli cannot be directly translated into cyanobacteria. For example, an engineered E. coli strain containing the 1-butanol pathway produced more than 30 g/L 1-butanol (Shen et al., Appl Environ Microbiol, 77:2905-2915, 2011), while a cyanobacterial strain with the same pathway produced only trace amounts of 1-butanol (Lan et al., Metab Eng, 13:353-363, 2011). Thus, there exists a need for construction of a biosynthetic pathway in cyanobacteria leading to significant production of a commodity chemical from CO2.